This invention relates to a heat insulating box that uses a urethane foam as a heat insulator.
A refrigerator box is a kind of heat insulating boxes and its general construction and method of production are described below with reference to "Handbook of Polyurethane Resins", published by the Nikkan Kogyo Shimbun, Ltd., pp.238-243 and pp.248-250 and "Plastics Market and Product Design-Electric and Electronic Devices", published by Plastics Age Co., Ltd. pp.58-67. FIG. 1 is a perspective view of a typical refrigerator box, and FIG. 2 is a cross section of that refrigerator box. Shown by 1 is an outer box, 2 is an inner box, and 3 is a urethane foam as a heat insulator. Outer box 1 may typically be produced by molding a painted or coated steel sheet into a predetermined shape (e.g. a gate in the normal or inverted position). Then, an inner box 2 also molded into a predetermined shape is combined with the outer box 1 and a liquid urethane stock from which a heat insulator urethane foam 3 is to be made is injected into the gap between the two boxes. The liquid urethane stock is subsequently foamed so that the outer box 1 is joined integrally with the inner box by means of the foamed urethane 3, which serves not only as a heat insulator but also as a member to retain the strength of the overall structure. Depending on the object of use, the outer box may be made of the same material as the inner box.
During foaming, the polyurethane which undergoes curing reaction will generate heat and the center of the urethane foam 3 will become as hot as 60.degree. C. and above. Hence, following the curing reaction of polyurethane, the urethane foam 3 will cool to shrink, developing a shrinkage stress. The stress causes distortion in the urethane foam 3 or inner box 2 and, if the material of the inner box does not have adequate strength, blushing or cracking will occur in the inner box. To avoid those problems, the material of the inner box must have good moldability, exhibit good adhesion to the urethane foam 3 and high resistance to the stress that may develop upon shrinkage at cold temperature; in addition, said material must satisfy other conditions such as high resistance to the impact of an article dropping in the refrigerator, as well as high resistance to chemicals that may contaminate the interior of the refrigerator such as edible oils and seasonings. Materials in current use that are said to satisfy those requirements include ABS resins (acrylonitrile-butadiene-styrene terpolymers), butadiene rubber containing styrene resins, and vinyl chloride resins (PVC).
As forming agents for the urethane foam 3, Freon CFC-11 (CCl.sub.3 F) is most commonly used since it has a good balance between heat insulating property, toxicity, safety, ease of handling and cost. CFC-11 is mixed in liquid form in the starting materials of polyurethane and during foaming, it is evaporated by the heat of reaction of urethane resin to form tiny cells. As time passes, CFC-11 will come out of the foam cells and diffuse to the ambient. Hence, the inner box 2 is subject to the action of CFC-11 not only during the injection of the starting materials of polyurethane but also by its diffusion out of the cells after completion of foaming. Styrene resins currently used to make the inner box have low resistance to CFC-11 and require a protective film or coat in order to avoid direct contact with the foam 3. Vinyl chloride resins (PVC) are less subject to the action of CFC-11 but, on the other hand, they have such low resistance to heat that they may deform upon exposure to heat that will be generated when the insulating material 3 undergoes reaction; furthermore, vinyl chloride resins are so low impact resistance that they are prone to crack. ABS resins are used most extensively today since they have a good balance between various properties such as moldability, stress relaxation upon shrinkage at cold temperature, impact resistance, solvent resistance and resistance to CFC-11.
With the recent concern over the depletion of ozone layers in the stratosphere, many countries have started to introduce global regulations on the production and consumption of Freons. CFC-11 is also within the class of materials under such regulation and the increasing difficulty in using it as a foaming agent for heat insulating polyurethane foams has necessitated the development of a substitute foaming agent. Available as such substitutes today are HCFC-123 (CHCl.sub.2 CF.sub.3) and HCFC-141b (CH.sub.3 CCl.sub.2 F) which are similar to CFC-11 in physical properties (e.g. boiling point and the latent heat of evaporation) and which are out of the scope of the applicable regulations.
However, compared to CFC-11, the substitutes HCFC-123 and HCFC-141b have great tendency to dissolve polymeric materials and their ability to swell and dissolve butadiene rubber containing styrene resins and ABS resins which are currently used as materials for making boxes is so great that using them as foaming agents in place of CFC-11 will not only lower the strength of boxes but also lead to their destruction or deterioration in appearance. If HCFC-123 and HCFC-141b are used as foaming agents for the polyurethane foam, ABS resins which are most commonly used today as box making materials suffer from the problem that they are so seriously attacked by the foaming agents that cracks will develop in the box. To avoid this problem, it has been attempted to increase the wall thickness of the box making materials by a great degree or to laminate them with a film that exhibits high resistance to HCFC-123 and HCFC-141b. In practice, however, these techniques have not proved to be completely satisfactory. Even if the wall thickness of the box making materials is increased, they will be affected by HCFC over time and, in the long run, the quality of the refrigerator box will deteriorate. Furthermore, thicker sheets either require a longer molding time to reduce the production rate or result in heavier box making materials and, hence, heavier refrigerator boxes. On the other hand, lamination with materials having high HCFC resistance is indeed effective in preventing the attack of HCFC by the necessary minimum thickness of box making materials. However, lamination is a separate step and leads to a higher cost of production. In addition, the cut portions of the box will not be laminated with HCFC-resistant materials and, hence, are subject to the adverse effects of HCFC. Hence, to prevent the HCFC attack, an extra means of protection is necessary, adding to the complexity of the production process, furthermore, the use of dissimilar materials in boxes renders it difficult to recycle them.
It is also common practice to improve the mechanical properties of box making materials by incorporating fillers such as glass fibers (GF) and carbon fibers (CF). However, both GF and CF are bulky with a fiber diameter of 5-20 .mu.m and a length of 100 .mu.m to a few millimeters and will deteriorate considerably the surface smoothness and aesthetic appeal of the shaped parts. Furthermore, those fibers deteriorate the moldability of box making materials.